Piezo Accelerometer Tutorial
Accelerometer Design
Assembly of a Vibration Sensor
This video is not supposed to be a real mounting instruction for an accelerometer but it is more to show the basic parts and how they interact with each other. The proper assembly methods have not been explained or mentioned. The parts joining methods must be suitable for the intended environment such as the temperature but also for the vibrating environment. We want to make sure that nothing becomes loose. A common practice is therefore welding or brazing to join the parts. For lower temperatures soft soldering or adhesive bonding may also be suitable. The external housing parts are normally welded together in order to provide a completely hermetic protection and continuous electrical shielding.
We have seen that the time constant (a very important characteristic) is depending on the resistance and the capacity of the piezo element. Regarding the capacity we cannot do much about during the assembly as it is given by design parameters. The resistance however is strongly dependent on the direct environment of the piezo element. It is very sensitive to pollution and humidity.
From this we deduct for the assembly that all the parts must be extremely clean and that there shall be no humidity captured in the housing.
Parts Design
Typical Accelerometer Parts
The details of parts design is of course an important portion of the know-how of a piezoelectric sensors manufacturer. Here we want to look at some detailed considerations which are generally known and published.
The sensor we are looking at is a compression mode accelerometer.
Base
The prime function of the base is to support the internal sensing element, and then to allow the mounting of the final sensor.
The size and shape of the base depends a lot of the purpose of the accelerometer.
The mounting surface needs to be flat and smooth especially when we intent to measure at high frequencies.
Insulation
The insulation rings are used for the electrical isolation of the piezo element from the housing.
There are designs without electrical insulation. In this case one pole of the piezo element is connected directlyto the housing.
Piezo Element
The piezo element in our case is a compression mode ring. It has in general an electrode (green) on top and bottom on which the electric charge appears when the ring is compressed in axial direction.
Inertial Mass
The inertial mass together with the piezo material determines the sensitivity of the accelerometer therefore
it is often made of a high density material like Tungsten.
All matching surfaces of the sensing element must be flat and smooth in order to transmit the vibration correctly.
Bolt
The parts are held together by a bolt.
In some designs the parts are just bonded or soldered together.
Cover
The cover together with the the base provides a hermetic enclosure of the sensing element. At the same time is a protection from all sorts of mechanical, thermal and electro-magnetic interference.
